Peripheral modules are used for forming an interface between a communications system and subscriber lines. A plurality of peripheral modules is combined in respect of functions and construction to form a line/trunk unit. Depending on the size of the communications system, the line/trunk units can in turn be combined to form line/trunk groups and are connected by way of voice and data multiplex channels with a switching network as the core component of a communications system. In a central control unit of a communications system, call processing is performed by the system software. The peripheral modules have a module control unit whose task consists in the assignment of occupied subscriber-side transmission channels to selected channels of multiplex channels. The control functions of the module control unit are principally implemented by software.
Half paths or double connections can come about as a result of errors in the module software or in the switching system. In the case of half paths, only one of two transmission channels is switched through for a voice connection between two subscriber lines, with the result that the subscribers concerned can either only hear or only speak and no conversation is therefore possible. In the case of double connections, a pair of subscriber lines is assigned in each case to one message source or to one message sink. Problems of this type occur particularly in the case of communications systems in which switching network multiplex channels are connected to subscriber-side transmission channels of the peripheral modules and are assigned by way of a flexible transmission channel assignment method.
These problems are also encountered when distributed control units are used for the connection or disconnection of connection paths between end points of a communications system.
Incorrect settings in the call-handling units can only be diagnosed and localized with difficulty, not least on account of their sporadic occurrence. Connection paths through the switching network and subscriber-side transmission channels which are to be selected are normally ascertained and stored centrally by database access routines. Settings messages are used to forward associated connection path settings data to executing distributed control units. The connection path settings data is entered in separate main memories by the executing control units. In the case of further settings being made, changeovers occurring or operations being initiated, the connection path settings data is for example read out in order to produce settings messages which are in turn transmitted to peripheral modules. In the execution chain outlined, connection path data can become corrupted, as a result of which the aforementioned fault situations can arise. This means, for example, that transmission channels indicated in a database as capable of being occupied have not yet been disconnected in a targeted manner for a new occupation but have been assigned again without targeted disconnection.
To date, problems of this type have been resolved following a report of a disrupted connection by a subscriber by localizing the cause of the malfunction to particular peripheral modules and powering down the assigned communications system and powering it back up again by a hardware restart. This type of procedure involves the disadvantage that when such a malfunction occurs all the subscriber lines within a communications system are affected by fault recovery measures of this type.